Ammonification

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Ammonification

Ammonification, in chemistry, is defined as the saturation with ammonia or any one of its compounds. Strictly speaking, ammonification refers to any chemical reaction that generates ammonia (NH₃) as an end product (or its ionic form, ammonium, NH₄⁺). Ammonification can occur through various inorganic reactions or due to the metabolic functions of microorganisms, plants, and animals. In the ecological context, however, ammonification refers to the processes by which organically bound forms of nitrogen occurring in dead biomass (such as amino acids and proteins) are oxidized into ammonia and ammonium. The ecological process of ammonification is carried out in soil and water by a great diversity of microbes and is one of the many types of chemical transformations that occur during the decomposition of dead organic matter.

Ammonification is a key component in the nitrogen cycle of ecosystems. The nitrogen cycle consists of a complex of integrated processes by which nitrogen circulates among its major compartments in the atmosphere, water, soil, and organisms. During various phases of the nitrogen cycle, this element is transformed among its various organic and inorganic compounds.

As with all components of the nitrogen cycle, the proper functioning of ammonification is critical to the health of ecosystems. In the absence of ammonification, organic forms of nitrogen would accumulate in large quantities. Because growing plants need access to inorganic forms of nitrogen, particularly ammonium and nitrate (NO₃^–), the oxidation of organic nitrogen of dead biomass through ammonification is necessary for maintenance of the productivity of species and ecosystems.

Ammonification

Nitrogen is one of the most abundant elements in the tissues of all organisms and is a component of many biochemicals, particularly amino acids, proteins, and nucleic acids. Consequently, nitrogen is one of the critically important nutrients and is required in relatively large quantities by all organisms. Animals receive their supply of nitrogen through the foods they eat, but plants must assimilate inorganic forms of this nutrient from their environment.

However, the rate at which the environment can supply inorganic nitrogen is limited and usually small in relation to the metabolic demands of plants. Therefore, the availability of inorganic forms of nitrogen is frequently a limiting factor for the productivity of plants. This is a particularly common occurrence for plants growing in terrestrial and marine environments and, to a lesser degree, in fresh waters (where phosphate supply is usually the primary limiting nutrient, followed by nitrate).

The dead biomass of plants, animals, and microorganisms contains large concentrations of organically bound nitrogen in various forms, such as proteins and amino acids. The process of decomposition is responsible for recycling the inorganic constituents of the dead biomass and preventing it from accumulating in large unusable quantities. Decom-position is, of course, mostly carried out through the metabolic functions of a diverse array of bacteria, fungi, actinomycetes, other microorganisms, and some animals. Ammonification is a particular aspect of the more complex process of organic decay, specifically referring to the microbial conversion of organic-nitrogen into ammonia (NH₃) or ammonium (NH₄⁺).

Ammonification occurs under oxidizing conditions in virtually all ecosystems and is carried out by virtually all microorganisms that are involved in the decay of dead organic matter. In situations where oxygen is not present, a condition referred to as anaerobic, different microbial decay reactions occur; these produce nitrogen compounds known as amines.

The microbes derive some metabolically useful energy from the oxidation of organic-nitrogen to ammonium. In addition, much of the ammonium is assimilated and used as a nutrient for the metabolic purposes of the microbes. However, if the microbes produce ammonium in quantities that exceed their own requirements, as is usually the case, the surplus is excreted into the ambient environment (such as the soil), and is available for use as a nutrient by plants, or as a substrate for another microbial process, known as nitrification (see below). Animals, in contrast, mostly excrete urea or uric acid in their nitrogen-containing liquid wastes (such as urine), along with diverse organic-nitrogen compounds in their feces. The urea, uric acid, and organic nitrogen of feces are all substrates for microbial ammonification.

One of the most elementary of the ammonification reactions is the oxidation of the simple organic compound urea (CO(NH₂)₂) to ammonia through the action of a microbial enzyme known as urease. (Note that two units of ammonia are produced for every unit of urea that is oxidized.) Urea is a commonly utilized agricultural fertilizer, used to supply ammonia or ammonium for direct uptake by plants, or as a substrate for the microbial production of nitrate through nitrification (see below).

Ammonium is a suitable source of nitrogen uptake for many species of plants, particularly those that live in acidic soils and waters. However, most plants that occur in non-acidic soils cannot utilize ammonium very efficiently, and they require the anion nitrate (NO₃⁺) as their source of nitrogen uptake. The nitrate is generally derived by the bacterial oxidation of ammonium to nitrite, and then to nitrate, in an important ecological process known as nitrification. Because the species of bacteria that carry out nitrification are extremely intolerant of acidity, this process does not occur at significant rates in acidic soils or waters. This is the reason why plants growing in acidic habitats can only rely on ammonium as their source of nitrogen nutrition.

Because ammonium is a positively charged cation, it is held relatively strongly by ion-exchange reactions occurring at the surfaces of clay minerals and organic matter in soils. Consequently, ammonium is not leached very effectively by water as it percolates downward through the soil. This is in contrast to nitrate, which is highly soluble in soil water and is leached readily. As a result, nitrate pollution can be an important problem in agricultural areas that have been heavily fertilized with nitrogen-containing fertilizers.

Humans and ammonification

Humans have a major influence on the nitrogen cycle, especially through the use of fertilizers in agriculture. Under nutrient-limited conditions, farmers commonly attempt to increase the availability of soil nitrogen, particularly as nitrate, and to a lesser degree, as ammonium. Rates of fertilization in intensive agricultural systems can exceed 446.2 lb/ac (500 kg/ha) of nitrogen per year. The nitrogen in the fertilizer may be added as ammonium nitrate (NO₄ NH₄) or as urea. The latter compound must be ammonified before inorganic forms of nitrogen are present, that is, the ammonium and nitrate that can be taken up by plants. In some agricultural systems, compost or other organic materials may be added to soils as a conditioner and fertilizer. In such cases, the organic nitrogen is converted to available ammonium through microbial ammonification, and nitrate may subsequently be generated through nitrification.

In situations where the rates of fertilization are excessive, the ability of the ecosystem to assimilate the nitrogen input becomes satiated. Although the ammonium produced by ammonification does not leach readily, the nitrate does, and this can lead to the

KEY TERMS

Decomposition— The breakdown of the complex molecules composing dead organisms into simple nutrients that can be reutilized by living organisms.

Eutrophication— A natural process that occurs in an aging lake or pond as that body of water gradually builds up its concentration of plant nutrients.

Leaching— The process of movement of dissolved substances in soil along with percolating water.

Nutrient— Any chemical that is required for life.

pollution of groundwater and surface waters, such as streams and rivers. Pollution of groundwater with nitrate poses risks for human health, while surface waters may experience an increased productivity through eutrophication.